1. Field of the Invention
This invention relates to a digital transmission system of the type having a plurality of regenerative repeaters which together form a digital link between a transmitter terminal station and a receiver terminal station, these terminal stations each comprising associated digital peripheral equipment, the transmitter peripheral equipment comprising means for dividing the bit stream to be transmitted into consecutive transmission frames having a fixed number of bit positions and for inserting at least one extra bit in a given bit position of each of the consecutive transmission frames, the transmitter peripheral equipment furthermore comprising a scrambler in which the bit stream thus obtained is combined modulo-2 with a scramble pulse pattern for generating a scrambled bit stream for transmission via the digital link and the receiver peripheral equipment comprising a descrambler for descrambling the scrambled bit stream derived from the digital link, this receiver peripheral equipment furthermore comprising means for selecting the extra bit inserted into the consecutive transmission frames.
2. Description of the Prior Art
A digital transmission system of the above mentioned type is known from, an article by Pierre E. Hervieux, entitled "RD-3 Long-Haul High Capacity Digital Radio" published in "Proceedings of the World Telecommunication Forum", Geneva 1975. In such a digital transmission system scrambling of the bit stream to be transmitted is used for facilitating the recovery of the clock pulse signals from the transmitted bit stream and to reduce the line spectrum and to make this spectrum more uniform.
However, a drawback of this scrambling is that, inter alia for monitoring the digital connection, extra bits inserted into the bit stream cannot be extracted from the transmitted bit stream until after this bit stream has been descrambled. Therefore, in practice, it is customary for economic reasons to monitor the digital connection in only one of several regenerative sections instead of in each regenerative repeater. However, this entails the drawback that if a regenerative repeater arranged for monitoring gives an alarm, a fault localization procedure must be performed thereafter to ascertain which of the regenerative sections preceding the monitored repeater does not function properly.